High density, low stress films, for example titanium nitride (TiN) films, for Back End of Line (BEOL) hard mask applications are used to reduce line bending for patterned low-k dielectric trenches as well as to improve low-k etching profiles by reducing line edge roughness and critical dimension (CD) variation.
Radio frequency (RF) power may be supplied to a predominantly DC plasma to increase the ionization rate of sputtered material in the plasma. The increased ionization rate allows for improved surface mobility of arriving atoms, resulting in a crystallographic film orientation that is advantageous to producing low stress and dense films. In addition, a high fraction of ionized material arriving at the substrate level allows for tuning of the incoming ion energy by means of a wafer pedestal impedance circuit.
The RF power is supplied in parallel at the chamber cathode (i.e. sputtering target). A magnetron is used for increased electron confinement at the target surface to improve plasma stability and sputter rate. An erosion track is established on the surface of the target where material is preferentially sputtered.
One drawback of using RF power supplied to the target in conjunction with magnetic confinement at the target surface is that the effectiveness of the RF power delivery is diminished as the target material erodes. The effectiveness of the RF power delivery is diminished as the target material erodes due to the fact that the physical distance from the surface of the magnet to the sputtering surface of the target decreases as the target thins. Additionally, the erosion track on the surface of the target further reduces the effectiveness of the RF power delivery as the erosion track can further confine the electrons and thus decreases the plasma density at the substrate level. As the magnetic confinement increases, the electrons supplied by the RF current cannot propagate as efficiently into the plasma as they follow the strengthening (as a function of erosion depth) magnetic field lines.
The reduction in RF power delivered to the plasma reduces the ionization rate as seen directly by a decrease in the current measured at the substrate support impedance circuit and indirectly by the impact on film properties. The advantageous film properties associated with higher ionization, such as low stress and high density, start to deteriorate as a function of target life due to the increase in magnetic confinement.
Accordingly, the inventors have provided improved methods and apparatus for processing a substrate.